New Zealanders have been told to move away from coastal areas of the North Island after three powerful earthquakes shook the region.
The national emergency agency warned of a tsunami threat with "unpredictable surges" expected along the east coast.
There were reports of chaos in some towns as hundreds of people tried to reach higher ground.
The South Pacific archipelagos of New Caledonia and Vanuatu have also been warned to prepare for dangerous waves.
Their coasts could see wave surges as high as 3m (10ft), while parts of South America - including Peru, Ecuador and Chile - were warned they could see 1m waves reaching their coasts.
The Hawaii-based Pacific Tsunami Warning Center said "tsunami waves have been observed", but as yet no damage has been reported.
'Bumper to bumper' traffic
New Zealand Prime Minister Jacinda Ardern posted on Instagram: "Hope everyone is ok out there."
The three earthquakes all came during early hours at magnitudes of over seven.
The most powerful, at 8.1 magnitude, struck at around 08:30 (21:30 GMT) near the uninhabited Kermadec Islands, 1000 km (621 miles) north-east of New Zealand.
While earlier tsunami warnings had been called off, the third quake prompted the National Emergency Management Agency to send out a new alert and tsunami sirens sounded in some areas.
"People near coast from the Bay of Islands to Whangarei, from Matata to Tolaga Bay, and Great Barrier Island must move immediately to nearest high ground, out of all tsunami evacuation zones, or as far inland as possible," the agency said.
It said "strong and unusual currents and unpredictable surges" were expected and the first wave may not be the largest. "Tsunami activity will continue for several hours and the threat is real until this warning is cancelled," the agency said.
"This has been a dynamic event, it is constantly evolving," Minister for Civil Defence, Kiri Allan, said.
"We are asking those who have moved to high ground or inland to stay where they are until an official all clear is given."
Local media reported gridlock in towns such as Whangarei and Whakatane as people tried to flee their homes, schools and workplaces.
In nearby Ohope town, resident Leslie Peake was quoted by the New Zealand Herald as saying the traffic was "bumper to bumper" with "huge queues of people evacuating".
Meanwhile local media posted footage of waves rolling into Tokomaru Bay.
New Zealand's civil defence department have advised people to walk or cycle to avoid getting caught in traffic as they evacuate to higher ground.
Just last week, New Zealand marked the 10th anniversary of the 6.3 magnitude earthquake that destroyed parts of Christchurch in the south Island, killing 185 people.
Meanwhile the tsunami threat has subsided for Hawaii and American Samoa.
Patrick Ti'a Reid, 37, was among those in American Samoa, situated some 3,300 km (2,000 miles) north-east of New Zealand, who had been told to evacuate to higher ground in an emergency bulletin.
"Alarms went off in our executive office building and we immediately took for higher ground," Mr Ti'a Reid, who works for the government in the US territory, told the BBC.
He said the tsunami of 2009, which was triggered by an 8.1-magnitude earthquake, was "very much vivid in many Samoans' minds".